How to join two parts of an armature
Go in Edit mode, select the 2 hip bones, then the torso bone, then ctrl P > Make Parent > Keep Offset. Now the torso bone is the parent of the 2 hip bones, and the Keep Offset option allows them to keep their position and not stick to their parent.

Also, you need to assign an Inverse Kinematic constraint to your calves, it allows to reverse the influence of the bones so that the legs are not controlled by the base anymore, but by controllers that you will create and assign as Targets.